1. Field of the Invention
This invention relates to a plasma addressing electro-optical device having a two-layer structure including two layers of an electro-optical cell such as a liquid crystal cell and a plasma cell, and more particularly to a driving method by which a plasma addressing electro-optical device of the type mentioned can be driven at a comparatively low voltage.
2. Description of the Related Art
An electro-optical device of the matrix type which employs a liquid crystal cell as an electro-optical cell such as, for example, a liquid crystal display device, conventionally employs, as commonly known means for assuring a high resolution and a high contrast, an active matrix addressing system wherein a switching element such as a thin film transistor is provided for each picture element and the switching elements are driven in a line sequential condition. However, according to the active matrix addressing system, it is necessary to provide a large number of semiconductor elements such as thin film transistors on a substrate. Accordingly, the active matrix addressing system is disadvantageous in that, when the substrate has a large area, the yield in production is low.
A solution to the disadvantage has been proposed by Buzak et al. and is disclosed in Japanese Patent Laid-Open Application No. Heisei 1-217396 (corresponding to U.S. Pat. No. 4,896,149 and No. 5,077,553) wherein a plasma switch is employed in place of a switching element formed from a thin film transistor or a like element. Now, general construction of a plasma addressing display device wherein a liquid crystal cell is driven making use of switches based on plasma discharge is described briefly. Referring to FIG. 3, the plasma addressing display device is shown and has a layered flat panel structure which includes a liquid crystal cell 101, a plasma cell 102 and a dielectric sheet 103 interposed between the liquid crystal cell 101 and the plasma cell 102. The plasma cell 102 is formed using a glass substrate 104 and has a plurality of parallel channels 105 formed on a surface thereof. The channels 105 extend, for example, in a direction along the rows of the matrix. The channels 105 are individually closed by the dielectric sheet 103 to define plasma chambers 106 which are individually separate from each other. Ionizable gas is enclosed in the plasma chambers 106. A convex portion 107 of the glass substrate 104 is disposed between each adjacent ones of the channels 105 and serves as a barrier rib for isolating the adjacent plasma chambers 106 from each other and also as a gap spacer for the plasma chambers 106. A pair of parallel plasma electrodes 108 and 109 are provided on a bottom surface of each of the channels 105. The pair of electrodes function as an anode and a cathode to ionize the gas in the plasma chamber 106 to produce discharge plasma. Such discharge area makes a row scanning unit.
Meanwhile, the liquid crystal cell 101 is constructed using a transparent substrate 110. The transparent substrate 110 is disposed in an opposing relationship to the dielectric sheet 103 with a predetermined gap left therebetween, and a liquid crystal layer 111 is filled in the gap. Signal electrodes 112 made of a transparent conducting material are formed on an inner surface of the transparent substrate 110. The signal electrodes 112 extend perpendicularly to the plasma chambers 106 and make column driving units. Picture elements in a matrix are defined at intersecting positions between the column driving units and the row scanning units.
In the display device having such a construction as described above, the plasma chambers 106 in which plasma discharge occurs are switched to be scanned in a line sequential condition while an image signal is applied to the signal electrodes 112 of the liquid crystal cell side in synchronism with such scanning to effect display driving of the display apparatus. If plasma discharge occurs in a plasma chamber 106, the potential of the inside of the plasma chamber 106 is put substantially uniformly to that of the anode A so that picture element selection of each row is performed. In other words, each of the plasma chamber 106 functions as a sampling switch. If an image signal is applied to each picture element while the plasma sampling switch is in an on state, then sampling holding is performed so that lighting or extinction of the picture element can be controlled. Also after the plasma sampling switch is put into an off condition, the image signal is held as it is in the picture element.
By the way, if plasma discharge occurs, then a difference between the anode potential and a potential at the signal electrodes is applied to the two-layer structure of the liquid crystal layer 111 and the dielectric sheet 103. Accordingly, the voltage actually applied to the liquid crystal layer 11 is equal to the difference divided in accordance with a ratio between the capacitance of the liquid crystal layer and the capacitance of the dielectric sheet. Taking such potential drop caused by the division in capacitance into account, a high voltage corresponding to ten times or so the voltage necessary for actual driving is added to the signal electrodes. Further, in order to prevent possible deterioration of the liquid crystal, it must be driven by ac current. Conventionally, the polarity of the driving voltage to be applied to the signal electrodes is inverted, for example, for each frame with reference to or with respect to an anode potential. Consequently, a power supply voltage corresponding to about twice the voltage necessary for actual driving of the liquid crystal is required, and since such inversion in polarity is involved in addition to the voltage drop by the division in capacitance described above, the actual driving voltage becomes very high. Consequently, a high load is provided in construction of a liquid crystal driving circuit. Thus, the conventional display device described above has a subject or problem to be solved in that it is difficult to form the device as an integrated circuit.